With increase in capacity of information processing in recent years, various information recording technologies have been developed. In particular, the surface recording density of an HDD using a magnetic recording technology is continuously increasing at an annual rate of approximately 100%. Recently, an information recording capacity exceeding 200 gigabytes/platter with a 2.5-inch diameter of a magnetic recording medium for use in an HDD or the like has been demanded, and in order to satisfy such a demand, an information recording density exceeding 400 gigabytes/square inch is required to be realized.
In order to achieve high recording density in a magnetic recording medium for use in an HDD or the like, a perpendicular magnetic recording type has been suggested in recent years. In a perpendicular magnetic recording medium used for the perpendicular magnetic recording type, an easy axis of magnetization of a magnetic recording layer is adjusted so as to be oriented in a perpendicular direction with respect to a base plate surface. The perpendicular magnetic recording type is more suitable for increasing recording density than a conventional in-plane recording type, since the perpendicular magnetic recording type can suppress a so-called thermal fluctuation phenomenon that a recording signal is lost due to a superparamagnetic phenomenon impairing thermal stability of the recording signal.
As a magnetic recording medium used for the perpendicular magnetic recording type, a CoCrPt—SiO2 perpendicular magnetic recording medium (see a non-patent document 1) has been suggested, since it has high thermal stability and a good recording property. This configures the magnetic recording layer to have a granular structure in which a nonmagnetic grain boundary is formed by segregating SiO2 between magnetic particles of Co crystals having an hcp structure (hexagonal close-packed crystal lattice) and grown continuously in columns, so that refinement of the magnetic particles and improvement in coercive force Hc are achieved together. It is known that an oxide is used for the nonmagnetic grain boundary (a nonmagnetic part between the magnetic particles), and it is suggested that either one of SiO2, Cr2O3, TiO, TiO2, and Ta2O5 is used, for example (patent document 1).
In the above perpendicular magnetic recording type, a perpendicular head of a single magnetic pole type is used as a magnetic head to generate a magnetic field in a perpendicular direction with respect to the magnetic recording layer. However, a magnetic field with sufficient intensity cannot be applied to the magnetic recording layer simply by using only the perpendicular head of a single magnetic pole type, since magnetic flux which has left a single magnetic pole end part immediately tries to return to a return magnetic pole on the opposite side. Therefore, an intensive magnetic field in a perpendicular direction is applied to the magnetic recording layer by providing a soft magnetic layer below the magnetic recording layer of a perpendicular magnetic recording disk to form a magnetic path in the soft magnetic layer. That is, the soft magnetic layer is a layer whose direction of magnetization is aligned according to a magnetic field (magnetizing filed) at the time of writing so that a magnetic path is formed dynamically.
As described above, the soft magnetic layer is a layer utilized at the time of writing, the direction of magnetization of which is aligned along a magnetic field at the time of writing. At the time of reading, however, a magnetic field that aligns the direction of magnetization is not applied to the soft magnetic layer, and therefore, in principle, the direction of magnetization is dispersed in irregular directions. The irregular directions are three-dimensional directions, and, if the direction of magnetization of the soft magnetic layer includes a perpendicular component, the component may be picked up as noise together with a signal of the magnetic recording layer at the time of reading by the magnetic head.
Therefore, regarding the soft magnetic layer, an AFC (Antiferro-magnetic exchange coupling) structure where the soft magnetic layer is split into two layers and a nonmagnetic spacer layer is interposed therebetween has been suggested and practiced. In the AFC structure, a lower layer and an upper layer are coupled and fixed by mutual attraction due to reversal of their directions of magnetization (exchange coupling). Therefore, the directions of magnetization of the respective soft magnetic layers at a non-application time of magnetic field become antiparallel to each other (parallel and opposite to each other), that is, they become parallel to a main surface of a base plate. This reduces the perpendicular components extremely so that the noise generated from the soft magnetic layer can be reduced.
The strength of exchange coupling in the AFC structure is expressed by an exchange coupling magnetic field Hex. Since a stronger Hex makes the direction of magnetization of the soft magnetic layer less susceptible to an external magnetic field, and accordingly formation of a flux path due to a leakage magnetic field can be prevented, the SNR (signal-noise ratio) can be improved.